The invention relates to a picture display device comprising a display tube having a display screen and an electron gun arranged opposite said screen, with a cathode centred along an electron-optical axis and a plurality of electrodes which jointly constitute an electron beam-producing part, said gun further comprising a tubular structure having an outer surface and an inner surface on which a resistance structure of a material having a high electrical resistance is provided, which structure constitutes a focusing lens.
Lately, research efforts have been made regarding the use of a main focusing lens constituted by a (particularly helical) high-ohmic resistance structure in display tubes in order to obtain a low spherical aberration. The resistance structure has at least one contact for supplying a static focusing voltage and at least one contact for supplying an anode voltage.
However, when a dynamic correction signal together with the static focusing signal is to be supplied, the desired effect which was to be expected on grounds of previous experiences with conventional focusing tubes does not occur, particularly not at scan frequencies exceeding 16 kHz. (Correction by applying a dynamic focusing voltage may be necessary, for example in the case of large deflection angles so as to keep the electron beam in focus throughout the screen. In the corners a focusing voltage is required which is different from that in the centre of the screen. This means that the dynamic focusing voltage is a signal varying with the scan frequency whose intensity is a function of the position of the electron beam spot on the screen.)